Plasmids as Replicons

Plasmids are autonomous self-replicating molecules of DNA (or very rarely RNA) (Fig. 16.01). They are not chromosomes, although they do reside inside living cells and carry genetic information. They are not regarded as part of the cell's genome for two reasons. First, a particular plasmid may be found in cells of different species and may move from one host species to another. Second, a plasmid may sometimes be present and sometimes absent from the cells of a particular host species. Thus, although plas-mids carry genetic information that may be expressed, they are not a constant part of the cell's genetic make-up nor are they needed for cell growth and division under normal conditions.

As discussed previously, replicons are self-replicating molecules of nucleic acid. Chromosomes, plasmids, virus genomes (both DNA and RNA) and viroids are all replicons. Strictly speaking, a replicon is defined by the possession of its own origin of replication where DNA (or RNA) synthesis is initiated. Thus, a replicon need not carry genes that encode the enzymes needed for its own replication, nor is it necessarily responsible for generating its own nucleotide precursors or energy. This means that plasmids and viruses are replicons, even though they rely on the host cell to provide energy, raw materials and many enzyme activities.

Plasmids may be regarded as living creatures in their own right. Just as worms wriggle through the soil and fish float in the sea, so plasmids proliferate inside their host cells. To a plasmid, the cell is its environment. So, although the plasmid is not alive in the same sense as a cell, neither is it merely part of the cell. In some ways plasmids are like domesticated viruses that have lost the ability to move from cell to cell killing as they go. Plasmids maintain some viral characteristics since the plasmid requires the host cell for replication enzymes, energy, and raw materials. Unlike viruses though, plasmids do not possess protein coats and since they cannot leave the cell they live in, they avoid damaging it. Viruses usually destroy the cell in which they replicate and are then released as virus particles to go in search of fresh victims. Plasmids replicate in step with their host cell (Fig. 16.02).When the cell divides, the plasmid divides and each daughter cell gets a copy of the plasmid.

It is easy to see how a virus that has lost the genes for its protein coat and/or for killing the host cell might evolve into a plasmid. Furthermore, certain genetic elements, such as P1 (see below), can switch between the two lifestyles and may live either as a plasmid or as a virus. It is also possible to imagine how a plasmid might pick up coat protein genes and/or killing functions and deregulate its DNA replication so evolving into a virus. Indeed, many plasmids possess host killing functions that they use to ensure that they are not lost by the host cell (see plasmid addiction, below). So which plasmid Self-replicating genetic elements that are sometimes found in both prokaryotic and eukaryotic cells. They are not chromosomes nor part of the host cell's permanent genome. Most plasmids are circular molecules of double stranded DNA although rare linear plasmids and RNA plasmids are known replicon Molecule of DNA or RNA that contains an origin of replication and can self-replicate

General Properties of Plasmids 427

FIGURE 16.02 Plasmids Replicate in Step with the Host Cell

When a bacterial cell is ready to divide, the replication machinery also duplicates the plasmid DNA. The two copies of the chromosome and two copies of the plasmid are then divided equally between the daughter cells. The replication of the plasmid does not harm the cell.

Plasmid DNA

FIGURE 16.02 Plasmids Replicate in Step with the Host Cell

When a bacterial cell is ready to divide, the replication machinery also duplicates the plasmid DNA. The two copies of the chromosome and two copies of the plasmid are then divided equally between the daughter cells. The replication of the plasmid does not harm the cell.

Cell

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